A Qantas 737 jet carrying 152 passengers and crew scraped its tail on the tarmac at Sydney Airport after the pilots tapped the wrong data into an iPad linked to the aircraft's computer, air safety investigators have found.

Shortly after the Boeing 737 departed Sydney bound for Darwin, a flight attendant who had been sitting in the plane's rear galley reported hearing a "squeak" during take off. It prompted the pilots to quickly run through a checklist to determine whether the plane had scraped its tail on the runway.

The pilots also contacted Qantas maintenance engineers and decided to fly onto Darwin on August 1 last year because they could not find any indication from instruments of a tail strike or damage to the plane.

After landing at Darwin, the captain discovered some paint damage and scrape marks to the 737 but found a cartridge containing a tail strike sensor was intact. It indicated that the plane's tail had "only just contacted the ground during take off", according to a final report from the Australian Transport Safety Bureau released on Monday.

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A Qantas 737 similar to the plane pictured scraped its tail on a runway at Sydney Airport in August last year. Credit:Paul Rovere

The pilots also later checked the performance figures on their iPad for the plane's take-off at Sydney. The first officer found the take off weight was 10 tonnes less than the plane's actual weight of 76.4 tonnes. This resulted in the pilots calculating the speeds for take off differently and changes to the aircraft's thrust setting.

Investigators from the ATSB found the tail strike was the result of two data entry errors in calculating the take-off performance figures for the aircraft.

"This resulted in the take-off speeds and engine thrust setting calculated and used for the take-off being too low," they said in the final report. "As a result, when the aircraft was rotated, it over pitched and contacted the runway."

Qantas has since made changes to pilots' pre-flight procedures, including additional cross checks once take-off data has been verified.

In one of the most serious tail-strike incidents in Australia, an Emirates A340 with 275 people on board scaped its tail on the tarmac at Melbourne Airport during take off in 2009.

Apart from leaving three long scapes on the tarmac, the Emirates jet tore through the fuselage alloy skin of its tail, left shards of metal on the runway, and hit two ground antennas and a strobe light near the end of the runway as it tried to climb.

Investigators later found the plane's take off weight had been significantly under estimated.

The air-safety bureau said on Monday that independent cross checks between pilots, improved automation systems and software design, and clear and complete flight documentation would all help prevent data entry errors.